The present invention relates to a method for controlling the formation and deposition of scale forming salts in aqueous media. The polymers disclosed herein also find use as part of a corrosion control system to maintain solubilization of the corrosion inhibitors such as zinc and phosphate compounds.
Deposits in cooling waters are classified in two categories--foulants and scales. Foulants usually result from suspended solids in the system and are high in organics. Scales on the other hand are hard, adherent mineral deposits that precipitate from solution. Foulants can usually be removed by pre-treatment of the cooling water, such as by filtration. It is the prevention of deposition of scale with which the present invention is primarily concerned.
The problems associated with mineral scaling in cooling water systems have been known for many years. In such systems water flowing around heat exchange equipment deposits mineral scale on the surface of the installation. This scale builds up in layers giving an insulation effect, reducing the heat transfer of the apparatus and also resulting in poor water circulation. Eventually this necessitates the shut-down of the unit to allow mechanical or chemical cleaning.
Depositions in lines, heat exchange equipment, etc., may originate from several causes. For example, the precipitation of calcium carbonate and calcium phosphate will form scale, but products of corrosion also result in a deposit of iron oxide salts. These are deposited as scales due to changes in temperature, pH, concentration, pressure and incompatible water additives.
The development of high pH and/or non-chromate corrosion programs has increased the potential for scale formation due to chemical precipitation. In particular, since most of the treatments currently used include phosphate and/or phosphonic acid compounds, the reversion of the polyphosphates and the organic phosphates plus the use of high alkaline operating conditions leads to the formation and deposition of highly insoluble calcium phosphate.
Although steam generating systems are different from cooling water systems, they share common problems relating to calcium phosphate and iron oxide formation and other mineral scale deposition. In this regard, the formation of scale and sludge deposits on boiler heating surfaces is the most serious water problem encountered in steam generation. Although current industrial steam producing systems make use of external treatments of the boiler feed water to reduce scale forming ions, those operations are not totally effective and do not provide adequate treatment since muds, sludge, silts and hardness-imparting ions are not treated thereby, and eventually are introduced into the steam generating system.
Accordingly, internal treatment, i.e., the use of solubilizing chemicals which have the ability to keep the scale-forming materials in solution at concentrations substantially higher than would be expected are used throughout the industry in an attempt to alleviate the problems encountered by such scale deposition. Such solubilizing chemicals include alginates, lignins, lignosulfonates, tannins, carboxymethyl cellulose materials, and synthetic polymers such as polyacrylates and polymethacrylates. Since most of the solubilizing chemicals used are effective for only one or two of the scale forming salts, it is necessary to use a mixture of chemicals in order to provide adequate scale prevention in these aqueous systems. Thus, depending upon the chemicals present, it may be necessary to employ polymer dispersants to control calcium sulfate and calcium phosphate with organo-phosphorous chemicals used to disperse calcium carbonate. Further, the presence of iron oxide or other materials may additionally reduce the action of the scale prevention chemicals being utilized.
Although the foregoing is directed for the most part to cooling water systems and steam generating systems, the same problems occur in scrubber systems and the like. Any aqueous system having calcium and magnesium cations and the exemplified anions, in particular phosphate, will experience the formation and deposition of the scaling salts.
A further need for water treatment polymers such as are disclosed herein arises in the case of corrosion control systems where there has been an increasing emphasis to the use of high pH and/or non-chromate agents such as those utilizing phosphate and/or zinc. These latter materials have limited solubility and it has been found necessary to add other chemicals to the systems in order to maintain the solubility thereof.